Bill,

I think my 54 radiators have a date marking as you describe: H-54 (I'll double check this tonight).

A Google search came up with a Corvette site that stated that the some Corvette Harrison radiators had a year/date code (nnA: 19nn; A for Jan, B for Feb, C for Mar, etc.). I do not really know what the 56H refers to (but it is clearly on BobB's radiator).

Tim